7/22/2019 Out of Step Protection Enhancements http://slidepdf.com/reader/full/out-of-step-protection-enhancements 1/6 OUT-OF-STEP PROTECTION ENHANCEMENTS D Hou and D A Tziouvaras Schweitzer Engineering Laboratories, Inc., USA ABSTRACT Power systems are subjected to a wide range of small or larger disturbances during operating conditions and they are designed to survive disturbances caused by faults, loss of a large generator, or line switching. The power system typically adjusts to these disturbances and continues to operate satisfactorily and within the desired bounds of voltage and frequency. Multiple system disturbances, however, could cause loss of synchronism between interconnected power systems that lead to loss of generation and load, and sometimes to wide-area blackouts. To mitigate the effect of these disturbances, it is common practice to provide controls called special protection systems that aid in maintaining system stability. In addition, properly designed power systems include out-of-step (00s) rotection systems that detect loss of angular instability and perform controlled network islanding to preserve stability within smaller networks. In this paper, we describe the application philosophy of s protection systems in transmission systems and discuss recent enhancements in the design of out-of-step tripping (OST) and blocking protection functions that improve the security and reliability of the power system. INTRODUCTlON Power systems in the US ave experienced a number of large disturbances in the last ten years, including the largest blackout, which occurred on August 14, 2003 in the Midwest and Northeast U.S. and impacted millions of customers. The July 2, 1996 and August IO 1996 major system disturhances also impacted several million customers in the Western U.S. All of these disturbances caused considerable loss of generation and loads and had a tremendous impact on customers and the economy in general. Typically, these disturbances happen when the power systems are heavily loaded and a number of multiple outages occur within a short period of time, causing power oscillations between neighboring utility systems, low network voltages, and consequent voltage instability or angular nstability. It is very expensive to design a power system to completely prevent very rare multiple outages and withstand their consequences. To mitigate the effect of these disturbances, it is common practice to provide controls called special protection systems or remedial action schemes. These special protection systems are designed to avoid voltage or angular instability and minimize the effects of a disturbance. Special protection systems include underfrequency and undervoltage load- shedding schemes, direct load and generation tripping, and many other schemes (1). Certain power system disturbances may lead to loss of synchronism between interconnected power systems. If such a loss of synchronism occurs, it is imperative that the system areas operating asynchronously are separated immediately to avoid wide-area blackouts and equipment damage. An effective mitigating way to contain such a disturbance is through controlled islanding of the power system using 00s protection systems. Controlled system separation is achieved with an OST protection system at preselected network locations. OST systems must he complemented with out-of-step blocking (OSB) of distance relay elements, or other relay elements prone to operate during loss of synchronism or unstable power swings. OSB prevents system separation from occurring at any locations other than the preselected ones. This paper illustrates the philosophy and application of OST and OSB schemes. In addition, we discuss the performance requirements of distance relays when faults occur during an 00s condition. While there are many challenges presented to distance relay element5 in correctly detecting faults after issuing an OSB, we selectively present two of them: security against external unbalanced faults, and correct faulted phase selection of internal line faults to trip only the faulted phase instead of all three phases. s PROTECTlON PHILOSOPHY The power system's response to a disturbance depends on both the initial operating state of the system and the severity of the disturbance. A fault on a critical element of the power system, followed by its isolation by protective relays, will cause variations in power flows, network bus voltages, and machine rotor speeds. Depending on the severity of the disturbance and the actions of protective relays and other power system controls, the system may remain stable and return to a new equilibrium state, experiencing what is referred to as a stable power swing. On the other hand, if the system is transiently unstable, then it will cause large separation of generator rotor angles, large swings of power flows, large fluctuations of voltages and currents, and eventually lead to a loss of synchronism between groups of generators or between neighboring utility systems. 2004 Schweitzer Engineering Labs INC. USA. Reproduced with kind permission
straightforward avoid tripping of any power system
elements during stable swings and protect the power
system during unstable or 00s conditions. When twoareas of a power system or two interconnected systems
lose synchronism, the systems must be separated from
each other quickly and automatically in order to avoidequipment damage and shutdown of major portions of
the power system. Uncontrolled tripping of circuit
breakers during an 00s condition could cause
equipment damage and pose a safety concern for utility
personnel. Therefore, a controlled tripping of certain
power system elements is necessary in order to prevent
equipment damage and widespread power outages, andminimize the e ffects of the disturbance.
Effect of s Condit ion on Transmiss ion Line
Relays an d Relay Systems
The loss of synchronism between power systems, or
between a generator and the power system, affects
transmission line relays and systems in various ways.Some relay systems, such a s segregated line differentialrelays, do not respond to an 00s condition. Directional
and nondirectional instantaneous overcurrent and
distance relays may operate during stable or unstable
power swings. Operation of these relays during a power
swing will cause undesired tripping of transmission
lines or other power system elements, thereby
weakening the system and possibly leading to cascading
outages and the shutdown of major portions of the
power system.
Instantaneous -pha se overcurrent relays will operate
during 00s conditions if the line current during the
swing exceeds the minimum pickup setting of the relay.
Likewise, directional instantaneous overcurrent relaysmay operate if the swing current exceeds the minimumpickup setting of the relay and the polarizing and
operating signals have the proper phase relationship
during the swing. Voltage-restrained or voltage-
controlled current relays used for backup protection of
generators are also prone to operate during power
swings or 00s conditions. Time-overcurrent relays
may o r may not operate, depending on the swing current
magnitude and the time delay settings of the relay.
Phase distance relays measure the positive-sequence
impedance for three-phase and two-phase faults. The
impedance measured by distance relays at a line
terminal during an 00s condition varies as a function
of the phase angle separation 6 between the twoequivalent system source voltages 2) . Distance relay
elements will operate during a power swing, stable or
unstable, if the swing locus enters the distance relay
characteristic. Zone distance relay elemen ts with no
intentional time delay are' most pron e to operate during
a power swing. Zone 2 distance relay elements used in
pilot relaying systems, such as blocking or permissive
type relay systems, are also prone to operate during,
power swings. Backup zone step distance relay elements
may or may not operate during a power swing,
.
depending on their time-delay setting and the time ittakes for the swing imped ance locus to traverse through
the relay characteristic.
It is important to recognize that the relationship between
the distance relay polarizing memory and the measured
voltages and currents plays a critical role in whether adistance relay will operate during a power swing.
Another important factor in modem distance relays is
whether the distance relay has a frequency-tracking
algorithm to track system frequency. Relays without
frequency tracking will experience voltage polarization
memory rotation with respect to the measured voltages
and currents. Furthermore, the relative magnitude of the
protected line and the equivalent system source
impedances is another important factor in the
performance of distance relays during power swings. If
the line positive-sequence impedance is large when
compared with the system impedances, the distance
relay elements may not only operate during unstahle
swings but may also operate during swings from which
the power system may recover and remain stable.
s Detect ion Method s and Types of Schemes
A short circuit is an electromagnetic transient process
with a short time constant. The apparent impedance
moves from the prefault value to a fault value in a very
short time (a few milliseconds). On the other hand, a
power swing is an electromechanical transient process
with a time constant much longer than that of a fault.
The rate of change of the positive-sequence impedance
is much slower during a power swing or 00s condition
than during a fault, and it depends on the slip frequency
of the 00s.The fundamental method for discriminating
between faults and power swings is to track the rate of
change of measured apparent impedance, because theimpedance measurement by itself cannot be used to
distinguish an 00s condition from a phase fault.
The difference in the rate of change of the impedance
has been traditionally us+d to detect an 00s condition
and then block the operation of distance protection
elements before the impedance enters the protective
relay operating characteristics. Actual implementation
of measuring the impedance rate of change is normally
performed though the use of two impedance
measurement elements together with a timing device. If
the measured impedance stays between the two
impedance measurement elements for a predetermined
time, then an 00s is declared and an OSB signal is
issued to block the distance relay element operation.
Impedance measurement elements with different shapes
have been used traditionally for the detection of OOS,including double blinders, concentric polygons, and
concentric circles.
To guarantee that there is enough time to cany out
blocking of the distance elements after an 00s is
detected, the inner impedance measurement element of
the 00s detection logic must he placed outside the
largest distance protection region that is to b e blocked.
The outer impedance measurement element for the 00s
detection has t t b e placed away from the load region to
prevent inadvertent OSB logic operation caused by
heavy loads.
s Tripping a nd Blocking Funct ions
There are basically two functions related to 00sdetection. The first function is the OSB protection
function that discriminates faults from stable or unstable
power swings. The OSB function blocks relay elements
prone to operate during stable and/or unstable power
swings to prevent system separation in an indiscriminate
manner. In addition, the OSB function must unblock
and allow relay elements to operate for internal faults
that occur during an 00s condition.
The second function, the OST protection function,
discriminates between stable and unstable swings and
initiates network sectionalizing or islanding during lossof synchronism. OST schemes are designed to protect
the power system during unstable conditions, isolating
unstable generators or larger power system areas fromeach other with the formation of system islands, in order
to maintain stability within each island by balancing the
generation resources with the area load.
To accomplish this, OST systems must he applied at
preselected network locations, typically near the
network electrical center, and network separation must
take place at such points to preserve a close halance
between load and generation. Where a load-generation
balance cannot be achieved, some means of shedding
nonessential load or generation will have to take place
to avoid a complete shutdown of the area.
As we discussed earlier, many relay systems are prone
to operate at different locations in the power system
during an 00s condition and cause undesired tripping.Therefore, OST systems must he complemented with
OSB functions to prevent undesired relay system
operations, to prevent equipment dam age and shutdown
of major portions of the power system, and to achieve a
controlled system separation.
Typically, the location of OST relay systems determines
the location where system islanding takes place during
loss of synchronism. However, it may be necessary in
some systems to separate the network at a location other
than the one where OST is installed. This is
accomplished with the application of a transfer trippingtype of scheme.
Uncontrolled tripping during 00s conditions can cause
damage to power system breakers due to transientovervoltages that appear across the breaker contacts
when switching a line that contains the electrical center
of a transmission system. The maximum transient
recovery voltage occurs when the relative phase angleof the two systems is 180 during the 00s condition.
To adequately protect the circuit breakers and ensurepersonnel safety, most utilities do not allow
uncontrolled tripping during 00s conditions and
restrict the operation of OST relays when the relative
voltage angle between the two systems is between -90and 90 degrees.
Application of OST and O S B F unc ti ons
While the 00s relaying philosophy is simple, it is often
difficult to implement in a large power system becauseof the complexity o f the system and the different
operating conditions that must he studied. The selection
of network locations for placement of OST systems can
hest he obtained through transient stability studies
covering many possible operating conditions. Themaximum rate of slip is typically estimated from
angular chang e versus time plots from stability studies.
With the above information at hand, reasonable settings
can be calculated for well-designed OST relaying
schemes.
The recommended approach for 00s relaying
application is summarized below:
Perform system transient stability studies to identify
system stability constraints based on many operatingconditions and stressed-system operating scenarios.
The stability studies will help identify the parts ofthe power system that impose limits to angular
stability, generators that are prone to go 00s during
system disturbances, and those that remain stable.
The results of stability studies are also used to
identify the optimal location of OST and OSB
protection relay systems.
Determine the locations of the swing loci during
various system conditions and identify the optimal
locations to implement the OS T protection function.The optimal location for the detection of the 00scondition is near the electrical center of the power
system. However, we must determine that thebehavior of the impedance locus near the electrical
center would facilitate the successful detection o f
00s.
Determine the optimal location for system
separation during an 00s condition. This will
typically depend on the impedance between islands,
the potential to attain a good loadgeneration
balance, and the ability to establish stable operating
areas after separation. High impedance paths
between system areas typically represent appropriate
locations for network separation.
Establish the maximum rate of slip between systems
for 00s timer setting requirements, as well as the
minimum forward and reverse reach settingsrequired for success%l detection of s conditions.
The swing frequency of a particular power system
area or group of generators relative to another power
system area or group of generators does not remain
constant. The dynamic response of generator control
systems, such as automatic voltage regulators, and
element after a time delay equal to UBD. The OSBrelay bit comes from the power swing detection logic,
indicating that the distance relay has already detected a
swing condition and blocked the distance elementsund er user-specified conditions.
FIGURE 3 Distance Elements With 00s Block and TimeDelayed S Q Reset
However, the UBD concept may he difficult to apply
when the system swing center moves as a function of
the source voltage magnitudes and the relative line and
source impedances during an .OOS, as FIGURE 4
shows.
X
FIGURE 4 The Location of 00s Center Is a Function of
Source Voltage M agnitudes
When the system 00s center falls within the line
section between stations R and S, the distance relays onthe line R-S would need a longer UBD than the relayson the line S-T so that they do not overreach for
external faults on the line S-T. However, if the swing
center moves to the line section S-T, then the UBD time
for the relays on this line should he longer than the
relays on the line R-S to achieve the same security for
faults on the line R-S. Therefore, it is difficult to apply
the UBD coordination time when swing center location
changes.
On parallel-line systems shown in FIGURE 5 it is
impossible to use the UBD time to coordinate with
external faults, because a fault internal to one pair of
relays on LI is external to the pair ofr elay s on L2.
67QF: Zone 2 67QR
6 7 QF Zone 27QF one 2
FIGURE Use POTT Scheme to Gain Protection SecurityDuring System 00s
To achieve security for external faults during system
OOS, one possible solution is to not reset the OSB hitfor the Zone 1 distance elements due to potential
